Process for the preparation of fine grain metal carbide powders and sintered articles therefrom

ABSTRACT

Fine grain metal carbide powders are conveniently prepared from the corresponding metal oxide by heating in an atmosphere of methane in hydrogen. Sintered articles having a density approaching the theoretical density of the metal carbide itself can be fabricated from the powders by cold pressing, hot pressing or other techniques.

This application is a division of our prior U.S. application Ser. No.429,269 filing date Dec. 28, 1973 now U.S. Pat. No. 3,932,594.

This invention relates in general to a process for the preparation offine grain, metal carbide powders. In one aspect, this invention isdirected to a process for the preparation of fine grain carbide powdersof metals such as tantalum, titanium tungsten, mixtures of tungsten andcobalt, and the like. In a further aspect this invention is directed tohigh strength and/or high surface area, sintered, metal carbidearticles.

The prior art has disclosed various method for producing metal carbide,structures, such as fibers, yarns, fabrics and the like. For example,one method is disclosed by B. H. Hamling in U.S. Pat. No. 3,403,008.This method consists of impregnating a preformed organic polymericmaterial, such as rayon, with a solution of a metal compound.Thereafter, the impregnated material is heated to evolve volatiledecomposition products, leaving a carbonaceous relic containing themetal in finely dispersed form. Finally, the relic is further heated to1000° to 2000° C. in a non-oxidizing atmosphere to form the metalcarbide. The resulting metal carbide has a shape similar to that of thestarting rayon material.

In U.S. Pat. No. 3,246,950 which issued to B. A. Gruber silicon carbidefibers of up to one inch in length are produced by the reaction ofsilicon monoxide and carbon monoxide in the vapor phase. Zirconiumcarbide fibers have also been produced by a process, as set forth inU.S. Pat. No. 3,385,669 to R. A. Clifton, et al, which comprisesreacting fibers of zirconium oxide with carbon in an inert environmentat an elevated temperature of about 1700° C.

U.S. Pat. Nos. 3,269,802 and 3,374,102 which issued to E. Wainer et alare directed respectively to the preparation of carbide structures andcross-linked carbon products containing metal carbides. The first patentconverts a carbonized material such as a filament or shaped product intoa metal carbide by heating in an atmosphere containing as a vapor thehalide or carbonyl of the carbide forming metal. The second patentimproves the strength and flexibility of the metal carbides of the firstWainer et al patent by converting only up to 25 mol percent of thecarbonized product to carbide.

All of the above mentioned patents are directed to metal carbide ormetal-carbide-containing structures or articles, such as fibers,textiles and shaped articles. None of these prior art patents teachesthe preparation of fine grain metal carbide powders which are sinterableinto metal carbide articles having a density approaching the theoreticaldensity of the metal carbide itself.

Accordingly, one or more of the following objects will be achieved bythe practice of this invention. It is an object of this invention toprovide a process for the preparation of fine grain, metal carbidepowders. Another object of this invention is to provide fine graincarbide powders of metals such as tantalum, tungsten, titanium, and thelike. A further object is to provide carbide powders of tungsten andcobalt. A still further object of this invention is to provide finegrain metal carbide powders which are sinterable into metal carbidearticles. Another object is to provide sintered metal carbide articleshaving densities approaching that of the metal carbide itself. These andother objects will, readily become apparent to those skilled in the artin the light of the teachings herein set forth.

In its broad aspect this invention is directed to a process for thepreparation of fine grain metal carbide powders and sintered articles.The metal carbide powders are prepared by a process which comprises thesteps of:

(a) impregnating a carbohydrate material with a compound of a metal;

(b) igniting the impregnated material product of step (a) to producefragile agglomerates of sub-micron size metal oxide particles;

(c) comminuting the metal oxide product of step (b) to produce finelydivided metal oxide powder having a mean particle size below one micron;and

(d) heating said metal oxide powder in an atmosphere of methane andhydrogen to convert said metal oxide powder to metal carbide powder.

The metal carbide powders prepared by the process of this invention arecharacterized by an exceptionally fine grain, i.e. averaging less than 1micron in size, which renders them ideally suitable for low temperaturesintereing. In most instances, sintered articles can be prepared whichhave a density close to the theoretical density of the metal carbide.

It has been observed that for the most part commercially available metalcarbides do not undergo sintering as easily and at the relatively lowtemperatures employed in the present invention. In contrast the metalcarbides prepared by the process of this invention, sinter essentiallyto the theoretical density of the metal carbide at relatively lowtemperatures.

The present invention is based upon the discovery that ultra-fine metaloxide powders can be prepared by a relatively uncomplicated andinexpensive method, wherein the powders that are produced by this methodcan be converted to the metal carbide and sintered essentially to theirtheoretical densities at relatively low temperatures, that is,temperatures significantly lower than those employed in the prior art.As indicated above, the method of the invention comprises contacting ametal compound with a carbohydrate material, igniting the material todecompose and remove the carbohydrate material and to insure conversionof substantially all of the metal compound to fragile agglomerates ofits metal oxide, followed by comminution of the thus formed agglomeratesto give the uniform, ultra-fine powders of this invention. Thereafterthe metal oxide is converted to the carbide, and if desired sinteredinto useful articles.

One of the advantages of this method is that the particles prepared bythis dispersive precursor method, and which make up the agglomerate, arealready of the proper size and uniformity. Thus, since the powdersproduced by this method have an extremely small and uniform particlesize, they can be sintered to form useful high stength pressed bodies atrelatively low sintering temperatures.

The metal compounds employed in the preparation of the metal oxides arecompounds of one or more metals whose ashes will remain as agglomeratesduring the ignition step, as opposed to densifying into solid coherent,large particles which would then require fracturing during comminutionrather than disruption of the aggregates as employed in the instantinvention. Examples of metals whose compounds can be employed, eithersingly or in mixtures thereof, include tungsten, titanium, tantalum,molybdenum, zirconium, hafnium, thorium, and mixtures thereof.

In practice, the metal compounds that are in the impregnation step arepreferably water-soluble compounds such as halides, oxyhalides,nitrates, sulfates, oxylates, and the like. Specific illustrativewater-soluble metal compounds include zirconyl chloride, zirconiumacetate, zirconium citrate, tantalum oxalate, thorium chloride, titaniumchloride, hafnium oxychloride, metal acetates, and the like.

A preferred method of impregnation is to immerse the carbohydratematerial in an aqueous solution of the metal compound(s). Afterimmersion the loaded material is removed from the solution and theexcess liquid is removed by centrifugation, squeezing, blotting, or thelike. Centrifugation is a preferred method for removal of the excessliquid. With liquid precursor compositions, e.g. solutions containingsoluble carbohydrates, the preferred method is to dehydrate and char themixture by heating at 80° to 100° C.

In the impregnation step, relatively inexpensive forms of carbohydratematerial can be used for this aspect of the invention. For instance,essentially solid materials such as paper, wood pulp, rayon and cottonlinters are useful, inexpensive materials than can be used for theimpregnation, as well as the other types of materials. The material canbe employed in a wide variety of forms. For example, they can be presentas fibers or spun from a viscous solution into a fibrous form. They canalso be extruded into filaments or be present as powders.

Alternatively, a salt solution of the oxide or oxides of interest can bemixed with starch or a solution of a soluble carbohydrate material suchas sugars, i.e., glucose or sucrose, or hydrolyzed starch. Hence theterm "contacting" as employed in the first step of the process formaking the metal oxide powders is intended to encompass bothimpregnation of solid materials and dissolution in liquid materials toform intimate mixtures of the two.

The second step in the production of the finely divided metal oxide isthe ignition of the carbohydrate material containing the metal compoundimpregnated therein. The ignition can be carried out simply by rapidlyheating the loaded material in air to a temperature sufficient to ignitethe carbohydrate material.

In many cases, the term "ignition" implies combustion accompanied byflame. However, flame is not necessarily present in all cases ofignition as desired in the present invention. The important factor is toeffect decomposition and removal of the carbohydrate material by amethod which produces fragile agglomerates of very small particles ofmetal compound(s) which are present in the carbohydrate interstices.Thus, the ignition step employed herein has as its object the oppositeeffect from the heating step employed in the process of the Hamling U.S.Pat. No. 3,385,915. In said process, it is desired to maintain thestructural integrity of the polymeric precursor in order to produce anobject having the same configuration as the polymer.

If ignition were employed in the process of the Hamling Patent, theresulting product would not inherently be a fragile agglomerate. Asindicated at column 7 lines 37 et seq. of that patent if the organicmaterial ignites instead of carbonizes, the melting point ofintermediate metal compounds might be exceeded or excessivecrystallization and grain growth may occur. Thus, during the ignitionstep of this invention, temperature preferably should not exceed thetemperature at which sintering to uniform relatively non-fragileagglomerates occurs. This temperature varies from one metal oxide toanother, but will normally be from about 900° to about 1300° C. Forzirconia, for example, it is desired not to exceed about 1000° C. toabout 1100° C.

After ignition, the metal oxide (which can be referred to at this pointas an "ash") is comminuted to break up the fragile agglomerates to formthe ultrafine powder of the invention. The comminution can be effectedby any convenient method which is suitable for this purpose.

By the term "comminution", "disruption" or "pulverizing" as usedthroughout the specification and appended claims is meant the separationof the individual particles which form the agglomerate without the needfor further subdivision or fracturing of the particles. Hence any methodwhich can achieve this end can be employed. However, from a practicalviewpoint it has been found that wet ball milling accomplishes thisbest.

The metal oxide powders obtained by the comminution step and which areemployed in the preparation of the metal carbide powders of thisinvention are those having a mean particle size below one micron. Thesefinely-divided metal oxide powders are ideally suited for conversioninto the fine grain metal carbides.

As previously indicated, the metal oxide material to be converted to thecorresponding carbide must be ultra-fine grained in order for theconversion into the carbide and subsequent sintered article to takeplace at low temperatures. Thus, it is essential that the metal oxideitself be a fine grain powder of average particle size of less thanabout 1 to 2 micron in order that the resulting carbide obtained aftersintering also maintains a fine grain size of the same magnitude.Carbide formation as described in the prior art usually involves hightemperature reaction, such as 1500°-2500° C. for a long period of time.This is especially true for the high melting carbides such as tantalumand niobium. Carbides, such as tungsten and molybdenum, may be formed atlow temperatures, but extended reaction times are usually required.

However, in the process of the present invention, it has been found thatconversion to the carbide can be achieved at considerably lowertemperatures of from about 800° C. to about 1200° C. and periods of timeof from about 1 to about 5 hours have been found suitable for conversionto the carbide. The exact temperatures employed will, of course, varywithin this range for the individual metal oxide.

In order to insure that the metal oxide is the desired ultra-fine grainsize, it has been found that a convenient way to monitor the activity ofthe starting metal oxides powder is the BET N₂ adsorption specificsurface area. When the equivalent spherical diameter (as calculated fromthe specific surface area by known methods) is less than 1000° A, andmore preferably less than 500° A the reaction of the oxides to thecarbide is reasonably rapid and complete and can be effected atrelatively low temperatures.

As indicated, conversion of the metal oxide to the metal carbide iseffected by heating the metal oxide powder in an atmosphere of methaneand hydrogen. In practice, it has been found that an atmosphere ofhydrogen containing from about 0.1 to about 10 volume percent methane iseffective for converting the metal oxide to the carbide. Concentrationsabove and below this amount can also be employed but are less preferred.A particularly preferred mixture is one containing from about 0.2 toabout 5 volume percent methane in hydrogen.

The sintered metal carbide objects of the invention can be prepared byconventional sintering techniques, except that the temperatures that canbe employed are significantly lower than those heretofore employed forsintering metal carbide powders. The metal carbide sintered bodies canbe hot pressed, or they can be cold pressed followed by heating tosintering temperature.

In another aspect, this invention is directed to the preparation ofmetal carbide mixtures, for example cobalt-tungsten carbides andnickel-tungsten carbides. In these cases the fine grain mixed oxideprepared by the precursor technique is carburized in a mixture of H₂ andhydrocarbon at a temperature and hydrocarbon partial pressure such thatone component forms a hard carbide, while the other component is reducedto finely divided metal. Subsequent hot pressing or cold pressingfollowed by sintering at elevated temperatures of this material usingknown techniques leads to dense bodies of tungsten carbide with a cobaltbinder. These materials have utility for hard facing, flame sprayingpowders, and the like. Variations in the process lead to powders havingdifferent properties, and consequently useful for specific applications.

One aspect of this invention is a powder of cobalt-tungsten carbide whenthe cobalt content can vary from 0-30%, in which the aggregrate size isallowed to increase to the maximum attainable by this precursor process.Such powders are useful in plasma spray application where the desiredpowder size is 325 mesh (U.S. Standard sieve size). In this case asoluble cobalt salt and soluble tungsten salt are mixed in a commonsolvent to give the desired ratio of Co to W in the final product. Asugar solution (such as corn syrup) or soluble starch in a volumeproportion of 1/4 to 1 to 4 to 1 (corn syrup to salt solution) is addedto the mixed salt solution. The amount of corn syrup is not critical butmost frequently a 1:1 volume ratio is used. The thoroughly mixedsolution is heated to drive off excess water and char the corn syrup.This process is continued until the whole mass is a rigidized char. Thechar is then broken up into a powder of 16-20 mesh and air oxidized in aforced air furnace to convert the mixture to a mixed oxide. In the caseof the cobalt tungsten mixed oxide, x-ray diffraction indicates theoxidized product to be CoWO₄ plus a small amount of free WO₃ and Cooxide. The material is then powdered by micromilling for a short timeand loaded into a furnace through which a stream of CH₄ -H₂ isintroduced. In practice, this is generally done in a rotating Al₂ O₃tube inside a furnace, so that the powder does not cake or plug thetube. A heat treatment at 900° to 1100° C. in an atmosphere consistingof 0.3 to 5% CH₄ in H₂ converted the mixed oxide (BET S=5 m² /g) to anintimate mixture of Co-WC. Surface area measurements on the finalproduct were in the range 0.1 to 5 m² /g. Densities of the powder,determined using a pyknometer in methanol ranged from 12.8 to 14.3 g/cc.

These Co/WC materials in powder form can be used to prepare dense bodiesof Co/WC composites by hot pressing, or by cold pressing followed byhigh temperature sintering.

A more active form of the above range of composition, e.g., Co/WC, canbe made by subjecting the powder as obtained from the air oxidation stepdescribed in the preceding paragraphs, to a ball milling step in wateror nonaqueous solvent, with ZrO₂ beads for several days. The suspendedsolids are charged with "Darvan-C", centrifuged under such conditionsthat particles 0.5μ and smaller remain suspended, and all largerparticles precipitate. The suspended mixed oxide is poured off theprecipitate, discharged by addition of a small amount of acetic acid,washed with water, acetone and dried. The BET surface area of the mixedoxide ranges from 10-20 m² /g.

This material can be carbided in 0.5 to 5% CH₄ in H₂ mixtures to giveCo/WC at temperatures ranging from 800°-1200° C. The preferredtemperature is approximately 1000° C., for times varying from 1 to 5hours, depending on the reaction apparatus and degree of mixing of thesolid and gas, to avoid gas channeling due to solid caking. The Co/WCpowder product showed densities ranging from 13 to 14.3 g/cc andspecific surface areas ranging from 1.33 m² /g and higher. This materialcould be hot pressed to near theoretical density, or cold pressedfollowed by sintering at high temperature.

As previously indicated the metal carbide powders prepared by theprocess of this invention are useful for a wide variety of applications.In addition to plasma spray applications the metal carbide powders areuseful in applications where strength, stiffness and hardness aredesired. For example, the metal carbide powders can be used in thepreparation of tool bits. The present invention permits the formation ofdense bodies at fabrication temperature much lower than obtained usingconventional ceramic technology. Not only is no binder needed, but thetemperature required is much lower and the times shorter to achieve highdensity bodies than in the current state-of-the-art. This results ineconomic advantage in the production of such materials. Other potentialuses include the use of carbides in metal that wet the carbide to yieldcomposites that have desirable abrasion or wear resistance properties.

The following examples are illustrative:

EXAMPLE 1 Preparation of a Sintered Tantalum Carbide Article

A Ta₂ O₅ powder, made by the precursor process by low temperature airoxidation of a charred tantalum oxalate-corn syrup solution, ballmilling, and centrifugal sizing gave an active powder with the followingproperties: Surface area (by N₂ BET) 73.8 m² /g corresponding to anequivalent spherical particle diameter of 89° A, pyknometric density5.87 g/cc (theoretical 9.05 g/cc). This powder was heated in a rotaryfurnace through which was passed a gas mixture of 5% CH₄ in H₂ (flowrate: 3 l/min H₂) at 1050° C. for 41/2 hours. The resultant TaC powderhas a density of 10.2 g/cc and specific surface area of 5.1 m² /g. Thismaterial was ground with a mortar and perstle sieved through 325 meshand cold pressed at 15 Tons/in² to give a pellet with a green density of4.54 g/cc (31.5% of theo.). No binder was needed. The green pellet washeated in H₂ for 6 minutes at 1900° C. to give a metallic sintered TaCbody having a density of 9.1 g/cc.

EXAMPLE 2 Preparation of Tantalum Carbide Powder

A sample of Ta₂ O₅, prepared by the precursor process, but not attendingto the many details in procedure that promote the preparation of anultrafine grain microstructure, had a specific surface area of 6 m² /g.This powder was heated in 5% CH₄ /H₂ for 35 hours at 1007° C. at a H₂flow rate of 1.38 l/min. The resultant powder when analyzed chemicallyshowed the presence of a high free carbon content (10.02%). X-raydiffraction of the sample showed the presence of well crystallized TaC.Attempts were made to remove the excess carbon by prolonged heating inH₂ at temperatures between 800° and 1300° C., but very little carbon wasremoved.

EXAMPLE 3 Preparation of Tantalum Carbide Powder

A Ta₂ O₅ powder prepared by the precursor technique had a specificsurface area of 45.2 m² /g. This material was heated in a quartz rotaryfurnace in an atmosphere of 5% CH₄ in H₂ (H₂ flow rate 3 l/min) atT=1018°-1070° C. for 51/3 hours. The resultant black material was shownby x-ray to be pure TaC. The powder had a pyknometric density of 11.0g/cc (in methanol) and a specific surface area of 9.3 m² /g, whichcorresponds rough to particles of 400°-500° A as spheres.

EXAMPLE 4 Effect of Temperature on Conversion to the Carbide

A sample of ultra fine grained Ta₂ O₅, having a specific surface area of59.6 m² /g was heated in 5% CH₄ /H₂ at a H₂ flow rate of 3 l/min for 2hours at a temperature of 1050°-1180° C. in a rotary furnace. Analysisof the product by x-ray diffraction showed crystalline TaC and Ta₂ O₅.

The sample was reheated in CH₄ /H₂ at 1175° C. for an additional 41/2hours in the same atmosphere composition and flow conditions, but thematerial did not undergo further conversion to TaC as shown by x-ray. Anadditional heat treatment for 7 hours under the same conditions at 1180°C. again showed no further conversion of unreacted Ta₂ O₅ to TaC. Thex-ray of the powder product showed that the initially broad lines of theTa₂ O₅ had become very sharp and narrow, indicating grain growth of theTa₂ O₅ phase, with the resultant loss reactivity. Thus, once thematerial becomes highly crystalline, further treatment will not giveadditional conversion. Hence, the efficacy of conversion to TaC isbetter when lower temperatures are employed for longer periods of timerather than the higher temperature, i.e., 1180° C. for shorter times.

EXAMPLE 5 Preparation of a Cobalt-Tungsten Carbide Sintered Article byCold Pressing

A cobalt-tungsten salt solution was prepared by mixing 250 ml ofammonium metatungstate (e.g.=2.82, 1.45 g WO₃ /ml) with 219 g CoCl.6H₂O, 25 ml concentrated HCl, 50 ml water, and 250 ml corn syrup. Thecomponents were thoroughly mixed and heated to gradually evaporate waterand decompose the entire mass into a cellular friable char to a finaltemperature of 200° C. The char was broken up to 16-20 mesh and ignitedin air at 800° C. for 45 minutes. X-ray diffraction of the resultingpowder showed a strong pattern of CoWO₄. The surface area was 0.39 m²/g.

The mixed oxide (93.2 g) was converted to the carbide by heating thepowder in an Al₂ O₃ tube rotating in a furnace in a gas streamconsisting gas was changed to pure H₂ when the temperature decreasedbelow 900° C. The resultant carbide (76.1 g) was sieved through U.S.standard screens to give 26.0 g (34.2%) 400 mesh and 50.1 g (65.8%) 325mesh powder. X-ray diffraction of the powder showed very crystalline WCwith weak cubic Co lines. The cell constant for the Co was 3,545° A(reported 3.434° A).

A portion of the Co/WC powder was cold pressed at 13.5 Tons/in² into abar with a green density of 4.4 g/cc. This bar was heated in H₂ to 1400°C. and held at 1400° C. for 30 minutes. During this heat treatment thebar became very metallic and shrunk in length approximately 25%. Thegeometrical density was 13.8 g/cc (97% theoretical).

EXAMPLE 6 Preparation of a Cobalt-Tungsten Carbide Sintered Article byHot Pressing

A CoWO₄ -WO₃ powder, prepared as described in the preceeding example wassubjected to an additional ball milling treatment for 6 days in waterusing zirconia beads. The slurry was suspended by adding "Darvan C" (5drops/0.5), centrifuging at 1200 rpm and retaining "fines" in thesupernatant. After separation, the "fines" were precipitated byde-charging the suspended material with acetic acid. The material waswashed with water and acetone, dried and powdered using a microblender.The CoWO₄ starting material used for conversion to the carbide had asurface area of 15.2 m² /g and a density of 10.93 g/cc. The carbideprepared by heating in CH₄ /H₂ stream at 812-916° C. for 3 hrs had apyknometric powder density of 14.5 g/cc. X-ray indicated a very strongpattern of WC with a very weak cubic Co pattern. Chemical analysis gave12.67% Co, 80.02% W, 4.88% carbide, 1.50% free carbon, indicating thecomposition WC₀.93.0.5 Co.

A bar of 400 mesh Co.WC powder, 1×1/4" was hot pressed at a temperatureof 1298°-1398° C. at a ram pressure of 1200 lbs (2.4 Tons/in²) in agraphite die in vacuum. The resulting Co/WC press had a geometricaldensity of 13.6 g/cc (95% theoretical) and a modulus rupture of 280,000psi when tested in 3-point loading in a Tinius Olsen testing machine, atroom temperature.

EXAMPLE 7 Preparation of a Cobalt-Tungsten Sintered Article by ColdPressing

Co/WC powder prepared by heating the oxide in CH₄ /H₂ as described inexample 6, was cold pressed into a 11/2"×1/4" at 13.5 Tons/in² to give abar with a green density of 4.4 g/cc. The sample was then heated in acarbon susceptor using an induction furnace at 1400° C. for 30 min,followed by rapid cooling. The bar was then squared off, polished andmeasured. The geometrical density was 13.81 g/cc, 96.5% of theoretical.

EXAMPLE 8 Preparation of Cobalt Tungsten Carbide

The impregnating solution was prepared by combining 3 l of 1.84 s.g.ammonium metatungstate (AMT) solution (775 g W/l) with 506 g CoCl₂.6H₂O. The cobalt salt was dissolved by stirring the solution. To thissolution was added 728 g paper pulp and impregnated for 7 days. Theloaded paper pulp was centrifuged to rid it of excess solution at 3000rpm in paper towels for 10 minutes, followed by air drying for 24 hours.The dried material was placed in trays and oxidized in a forced airfurnace at 400° C. to give a greenish blue oxide with a specific surfacearea 1.33 m² /g.

The oxide material was placed in an alumina boat and then the boatplaced in a tube furnace through which H₂ was passing at a flow rate of3.0 l/min. The temperature was increased to 705° C. and CH₄ was added soas to give a 5 v/o CH₄ mixture in H₂. The temperature was raised to1050° C. and held at 1030°-1050° C. for 4 hours. The powder was given a40 minute post treatment in pure H₂ at 1030° C. and then the system wascooled to room temperature.

The cobalt/tungsten carbide sample powder was divided into 2portions--top and bottom for chemical analysis to determine thehomogeneity of the preparation. The analytical data are as follows:

Top: W 88.30, carbide 5.81, free carbon 0.00, oxygen 0.075%

Bottom: W 88.12, carbide 5.87, free carbon 0.00, oxygen 0.066%.

Although the invention has been illustrated by the preceding examples itis not to be construed as being limited to the materials employedtherein, but rather, the invention relates to the generic area ashereinbefore disclosed. Various modification and embodiments can be madewithout departing from the spirit and scope thereof.

What is claimed is:
 1. A process for producing sintered metal carbidearticles which comprises the steps of:(a) impregnating a carbohydratematerial with a compound of a metal; (b) igniting the impregnatedmaterial product of step (a) to produce fragile agglomerates ofsub-micron size metal oxide particles; (c) comminuting the metal oxideproduct of step (b) to produce finely divided metal oxide powder havinga mean particle size below one micron; (d) heating said metal oxidepowder in an atmosphere of methane and hydrogen to convert said metaloxide powder to metal carbide powder; (e) compressing and sintering saidmetal carbide powder into a shaped article.
 2. The process of claim 1wherein said metal is tungsten, titanium, tantalum, molybdenum,zirconium, hafnium, thorium or mixtures thereof.
 3. The process of claim2 wherein said carbohydrate material is cellulosic.
 4. The process ofclaim 3 wherein the cellulosic material is rayon.
 5. The process ofclaim 2 wherein said carbohydrate material is soluble carbohydrate. 6.The process of claim 5 wherein said soluble carbohydrate is a sugar. 7.The process of claim 5 wherein said soluble carbohydrate is a starch. 8.The process of claim 2 wherein said metal is a mixture of tungsten andcoblat.
 9. The process of claim 2 wherein said metal is a mixture oftungsten and nickel.
 10. The process of claim 2 wherein said heating isconducted at a temperature of from about 800° to about 1200° C.